In the treatment and clarification of raw water in a clarifier, such as a recirculating solids contact clarifier, or any other sedimentation apparatus employing one or more liquid treatment zones, suspended solid particles within the raw water act as "seed" or nuclei. Newly formed precipitates adhere to such particles creating a smaller number of larger, more dense and easily settled particles. Previously formed precipitates act as the "seed" to speed the reaction between the incoming raw water and treatment chemicals added to the system.
Examples of such apparatus are disclosed in U.S. Pat. Nos. 3,473,665 and 3,951,806, both of which are assigned to the same assignee as the present invention. These apparatus recirculate settled precipitates or "sludge" upwardly into an uptake or reaction zone for mixing with the incoming raw water and chemical treating agents which can be introduced within the reaction zone in desired concentrations.
In order to ensure an optimum rate of recirculated precipitates to accomplish coagulation and clarification, a variety of variables within the system must be monitored and maintained at desired levels. The primary variables which must be so controlled include the concentration of suspended solids within the reaction zone; the speed of a recirculator member typically positioned within the reaction zone; the type, concentration and amount of chemicals being added within the reaction zone; the frequency and duration of sludge removal or "blow off" from the separation or sedimentation zone as well as sludge inventory and density. Additional variables include, among others, the raw water inlet flow rate, composition and temperature.
Typically, set up and adjustment of existing recirculation solids clarifiers is conducted manually. Samples are periodically manually drawn from respective portions of the clarifier and analyzed to determine the amount of suspended solids concentration.
Based on the laboratory results, an attendant, relying primarily on experience, makes adjustments to one or more variables such as the speed of the recirculator member, the chemical input, the sludge blow off frequency and duration as well as the inlet and outlet flow rates. Following a prescribed time period, which can be up to twelve hours, samples are again drawn from respective portions of the clarifier for lab analysis.
The above described process is not very reliable due to a number of factors including the level of experience of the operator, the frequency of samples drawn, the accuracy and interpretation of the lab analysis, and the time in which it takes to determine and correct a potential problem. Returning the clarifier to optimum operating conditions following a change in the system can take a substantial amount of time during which the flow exiting the clarifier will not be up to prescribed standards.
It therefore would be desirable to provide a method and apparatus including an automatic feedback control system for a clarifier, such as a recirculating solids contact clarifier, that maintains steady-state operation of the clarifier by accurately measuring the concentration of suspended solids at designated points in the clarifier and automatically adjusting clarifier variables to maintain optimum operating conditions with no substantial delay despite any changes that may occur in the raw water inlet flow rate, composition, or temperature.